In the field of large area electronics, low cost but high performance sensors are one of the most targeted applications. These sensors can be used for a variety of acquisition functions such as thermal measurements, x-ray detection, and pressure sensing, to name a few. These devices are used in diverse fields including medical, environmental, security and industrial, amongst others. In particular, biometric security has attracted a great deal of attention in the recent years and, at the forefront of this field, fingerprint technology is the dominant technology. The fine patterns formed by ridges and valleys on the finger's skin can be mapped by sensing arrays of high resolution. These sensing arrays have been used for a number of years in this field, but they vary in basic operating principles. Some sensors utilize heat signals, while others electrical or optical signals. Accuracy levels are limited by the physical principles used to read fingerprint patterns (i.e., optical, capacitive, pressure, etc.) and most have not yielded the level of accuracy required for biometric security purposes. Furthermore, immunity to environmental variables such as dirt or humidity is also important when performing a fingerprint scan.
Development and commercialization of these sensing arrays are usually dictated by the cost of the processing technology used to build them as well as their sensing accuracy. Because these sensors are built over a large area, selecting a suitable technology that can yield the appropriate level of accuracy at an acceptable cost is often difficult.
The most efficient and accurate sensing arrays are based on active principles. Active sensors quantify a specific physical parameter response to a given stimulus. One of the most promising methods is the active thermal principle. In particular, active thermal sensors measure an object's heat conductance for a given heating stimulus. Examples of sensors of this type are disclosed in U.S. Pat. No. 6,091,837 to Dinh, entitled “Sensor for Acquiring a Fingerprint Image Based on Heat Transfer” (hereinafter “Dinh I”) and WO 2006/033582 A1, also to Dinh, entitled “Apparatus for Fingerprint Sensing and Other Measurements” (hereinafter, “Dinh II”), the entirety of each of which is hereby incorporated by reference herein. The response to the stimulus is measured by each of the sensing sites within a sensor array. The thermal response of an element is in part a function of the stimulus provided, i.e., the larger the stimulus, the larger the response. Sensing sites are heated by application of an electrical current to the site.
Recent advances in lower cost semiconductor electronics, such as high performance polycrystalline silicon (“polysilicon”) thin film transistors (TFTs), have enabled the implementation of accurate sensing arrays at a reduced cost. Pairing this device technology with the active thermal principle for fingerprint scanning can provide advantages such as low profile devices, improved ruggedness and accuracy. Use of this technology also provides the ability to integrate control circuitry on the same panel as the sensing array, further reducing cost and increasing integration levels. One of the main drawbacks of using these lower cost technologies, however, is their limited device performance when compared to conventional, single crystalline electronics. Even though material properties have improved dramatically in recent years, improvements are still needed. For example, it is important to acquire an electric signal that is free of electrical noise (i.e., a signal that has good signal-to-noise ratio). This ratio eventually determines the accuracy of the system and ultimately its ability to detect, for example, correct fingerprints. Thin film devices formed using polycrystalline material can generate high amounts of electrical noise, particularly when using a large stimulus (e.g., current) to obtain a high response. This makes the task of obtaining a good signal to noise ratio difficult.
Sensing architectures having improved device performance are desired.